This invention generally relates to the cleaning of heat exchanger vessels, and is specifically concerned with a system and method for vertically flushing the secondary side of a nuclear steam generator during a pressure pulse or other shock-wave type cleaning operation.
Methods for cleaning the interior of the secondary side of a nuclear steam generator by means of shock waves introduced into water are known in the prior art. In all of these methods, the nuclear steam generator is shut down and drained. Next, enough de-mineralized water is introduced into the secondary side to completely submerge the tubesheet and the bottom ends of the bundle of heat exchanger tubes mounted therein. Shock waves are then introduced into this water to loosen sludge and debris that accumulates around the top side of the tubesheet and the bottom ends of the heat exchanger tubes. Such shock waves may be generated by directly introducing a pressurized pulse of an inert gas within the water to produce an explosive, omnidirectional shock wave that impinges against all the heat exchanger components that are submerged within the water present in the secondary side of the generator. Alternatively, these shock waves may take the form of forceful fountains of water which erupt from the surface of the water collected within the secondary side and forcefully slap against the sludge-collecting spaces between the heat exchanger tubes and support plates to clean them. In still another type of shock wave cleaning process, a water cannon powered by pressurized pulses of gas is used to generate high velocity bursts of water below the water level which forcefully impinge against collected sludge and debris, thereby loosening and removing it. In all three methods, a gas operated pressure pulse generator is used to generate the shock waves which loosen the sludge and debris, either directly as illustrated for example in the pressure pulse cleaning techniques disclosed in U.S. Pat. Nos. 4,655,846 and 4,699,665, or indirectly through water slap and water cannon techniques as illustrated in U.S. Pat. Nos. 4,756,770 and 4,773,357, respectively. In all of these techniques, the same water that is used to propagate the sludge-loosening shock waves is continuously recirculated through the hand-holes located at the bottom of the steam generator and filtered in much the same manner as an ordinary pool vacuum in order to entrain and remove the sludge and other debris loosened by the shock waves.
Since their inception, such shock wave cleaning techniques have shown themselves to be a very promising way in which to remove the stubborn deposits of sludges which tend to accumulate not only on the upper surface of the tubesheet, but in the small annular spaces between the support plates and the heat exchanger tubes which are present inside the secondary side of such generators. However, the applicants have found that all of these shock wave cleaning techniques have fallen short of fulfilling their full potential due to the lack of effectiveness of the water circulation employed to entrain and remove the loosened particles of sludge and debris out of the secondary side of the steam generator. In all of these techniques, water is circumferentially circulated around the bottom of the secondary side just above the tubesheet by pumps which simultaneously inject and withdraw the water out through the sludge lancing ports of the steam generator. While such a flow of water effectively removes sludge and debris directly in front of the discharge and withdrawal ports of the re-circulation system and around the edges of the tubesheet where the concentration of heat exchanger tubes is at its lowest density, the applicants have found that the currents generated by such re-circulation systems are ineffective in sweeping the sludge and debris which accumulates on and around the central portion of the tubesheet where the density of the bundle of heat exchanger tubes is greatest. If this dislodged sludge is not removed from the center portion of the tubesheet, the fine particles which constitute such sludge are capable of settling onto the tubesheet and densely depositing themselves into the crevice regions between the tubesheet and the legs of the heat exchanger tubes mounted therein, thereby defeating one of the primary purposes of the cleaning operation. Of course, such sludge can be removed by conventional sludge-lancing techniques. However, the addition of another major step in the cleaning operation protracts the time necessary to complete the cleaning operation by as much as a half a day. This is a significant drawback, as a utility typically looses over $500,000 in revenues per each day of down-time.
Clearly, there is a need for an improved recirculation system for use in conjunction with a shock wave cleaning operation of a nuclear steam generator which effectively entrains and removes all the sludge and debris loosened by the shock waves without adding any significant amount of time to the cleaning operation. Ideally, such a re-circulation system should improve the efficiency of the cleaning operation without adding any significant expenses in set up time or equipment costs. Finally, the new re-circulation system should be compatible with all types of shock wave cleaning techniques, including pressure pulse, water cannon and water slap cleaning techniques.